This invention relates to a process for the purification of effluents by which the separation, working up and disposal of the sludges accumulating in mechanical and/or biological purification plants are improved.
In biological purification plants, the biological purification of effluent is carried out by means of microorganism. The organic constituents of the effluent are biologically eliminated. Under the conditions used, the microorganisms multiply to a praticularly marked extent, the organic impurities being assimilated by the microorganisms in the so-called activated sludge basins of the purification plants. Accordingly, some of the biomasses, consisting mainly of bacteria, have to be continuously removed from the activated sludge basins in the form of surplus sludge in order to maintain the optimal conditions for microbial effluent purification. For this reason, biomasses accumulate worldwide in extremely large and ever increasing quantities in the biological purification of industrial and communal effluents. In the Federal Republic of Germany alone, around two million metric tons (expressed as a dry weight) per year of these protein-containing biomasses are either dumped or burned. The necessary removal of the water from the activated sludge is a problem because, under the sedimentation conditions normally applied in practice in the intermediate and final settling tanks of purification plants, the activated sludge to be removed contains only about 1% by weight of microbial dry mass. The solids content can only be increased to around 3 to 9% by weight, depending on the type of sludge, by machine or static thickening. Even where polyelectrolytes are added, the solids content can only be increased to between 8 and 15% by weight by filtration or centrifuging.
Even in these low concentrations, the activated sludges have a pronounced gel structure and a relatively high viscosity due to the pronounced chemical and physical binding of the water to the microorganisms. For this reason, normal filtration or centrifuging is impossible without special treatment. Removal of the water is also complicated by the fact that the bacteria cells attract one another and form common, slimy shells, resulting in the formation of tacky flakes. In practice, therefore, inorganic sludges (for example of lime, asbestos flakes or insoluble metal oxides) are added to the surplus activated sludge in substantially the same to twice the quantity in order to facilitate removal of the water on an industrial scale by means of filter presses. In this way, a filter cake having a water content of around 50% by weight is obtained. FOr burning, however, the surplus activated sludge should have an organic matter content which is as high as possible. This is because the removal of water from the accumulating sludges, which have a relatively low concentration even where expensive organic polyelectrolytes are used as filtration aids, requires far more energy than can be obtained in heat equivalents during burning.
The problems outlined will increase in significance in the coming years. According to estimates of the Federal Ministry of the Interior of the Federal Republic of Germany (1975 Waste Economy Program of the Federal Government; Environmental Letter 13, 1976), the annual accumulation of treated sludge will have increased by 1985 to around 50 million cubic meters from communal plants plus another 30 million cubic meters from industrial plants, which for a water content of 95% represents approximately 4 million metric tons of dry purified sludge per year.
The sedimentation of activated sludges in the intermediate and final settling tanks must also be improved in regard to purification capacity and the often problematical working up, if the officially stipulated limits are to be observed. In addition, there is an economic need to optimize the water separation processes in order to save at least some of the flocculating agents which are currently being used in large quantities. Finally, either the cost of burning the surplus purified sludge will have to be reduced or, better still, new ecologically acceptable uses will have to be found. It would be preferred if such new uses were directed to the industrial recycling of the activated sludges which consist mainly of high-grade proteins, nucleic acids, enzymes and other valuable organic compounds.
The problems as stated above are solved to a large extent by the process according to the present invention.